Cell surface antigens in the preimplantation mouse embryo serve as markers of gene activity during differentiation. Using the H-Y antigenic system as a model, we plan: (1) to modify and improve existing methods for the identification of cell surface antigens on preimplantation mouse embryos. This includes (a) adapting the 'unlabeled antibody' immunoperoxidase lableing technique for the detection of embryonic cell surface antigens and (b) production of monoclonal H-Y antibody. (2) To apply these methdos to the study of temporal changes in antigenic appearance on cells of the preimplantation mouse embryo during various stages of development, and (3) To determine whether these serologically detectable embryonic antigens are, in fact, functioning in the differentiating process. Two cell embryos will be cultured in media containing antiserum directed against H-Y in order to determine if further differentiation of the embryo could be blocked. Experiments of this nature are crucial in determining, in fact, whether or not embryonic cell surface antigens are functionally involved in cell-to-cell recognition and differentiation in normal embryonic development.